For exhaust gas aftertreatment, e.g., of diesel exhaust gases of diesel vehicles, the so-called “Selective Catalytic Reduction Method” (SCR method) for the removal of nitrogen oxides (NOx) has been used. For this purpose, in appropriate reactors in the vehicle, the reducing agent ammonia (NH3) is recovered, which selectively reacts with nitrogen oxides, among other things, to form molecular nitrogen.
In the most widespread SCR method in the vehicle field, a urea-water solution (UWS) is sprayed into the exhaust tract, the UWS decomposing by urea hydrolysis to NH3 and carbon dioxide (CO2). To support the hydrolysis, there is usually a so-called “hydrolysis catalyst” in the flow direction of the exhaust gas, after the the injection point.
A urea-water solution (UWS), for the purpose of understanding the present invention, is understood to be a liquid or solution containing at least urea and water. In the case of the conventionally used 32.5% solution, however, the low antifreezing property (eutectic at −11° C.) may be disadvantageous, e.g., in vehicle applications.
In addition, it may be disadvantageous when using an UWS that it is decomposed by elevated temperatures beginning at 60° C. to 80° C. This generates ammonia (NH3) and carbon dioxide (CO2) from urea. The result may be that the solution, which is frequently originally a 32.5% solution, has a lower, generally unknown urea concentration because of this thermoses. However, without knowledge of the urea concentration, an optimal nitrogen oxide control of the exhaust gas cannot be ensured. This is so because if too much urea is added, poisonous ammonia is emitted, and if too little urea is added, a relatively large quantity of NOx is emitted.
At this time, discussions are already being held about a direct or indirect identification of the changing urea concentration of the UWS. The measuring methods appropriate for this, however, may only be able to be implemented at comparatively high cost and manufacturing complexity.